Reversible (ischemic) and mild-moderate irreversible (nonischemic) thallium defects represent regions with viable myocardium. Since the mere presence of viable myocardium does not necessarily indicate ischemic myocardium, we investigated whether stress-induced reversible thallium defects are more accurate in predicting recovery of function after revascularization compared to nonischemic but viable myocardium (mild-moderate irreversible thallium defects). We studied 11 patients with chronic coronary artery disease who underwent pre- and post-revascularization quantitative stress-redistribution-reinjection thallium, gated magnetic resonance imaging (MRI) and radionuclide angiography. Mean LV ejection fraction at rest increased from 31 plus/minus 8% pre-revascularization to 38 plus/minus 11% post-revascularization (p<0.05). Two to four MRI and thallium transaxial slices were matched and analyzed per patient. For each slice, pre- and post-revascularization systolic wall thickening was visually assessed in 5 regions by MRI. From a total of 137 revascularized regions, 62 regions had abnormal wall motion pre-revascularization, of which 42 improved post-revascularization and 20 did not improve. Among the 42 abnormal regions which improved post-revascularization, 21 (50%) exhibited reversible thallium defects and only 2 had mild-moderate irreversible defects. In contrast, of the 20 abnormal regions that did not improve post-revascularization, 14 (70%) demonstrated mild-moderate irreversible defects and only 3 had reversible thallium defects (p<0.0001). These data suggest that when performing stress studies, most mild-moderate irreversible thallium regions represent an admixture of viable (nonischemic) and scarred myocardium that may not improve post-revascularization. On the other hand, the identification of reversible thallium defect on stress in an asynergic region more accurately predicts recovery of function after revascularization.